All the surrounding towns are booked up as well and the trains do not run early enough to do it that way. The prices are all about supply and demand, it is the same if you go to New York during the high season. But in Pamplona you only have 10 days of a high season but 2.5 million people want to go and party in a town that only has less than 3000 hotel beds. So your options are either sleep out in a park and maybe get robbed or wet, or get some rooms somewhere. The trick to get a good deal on rooms is to be very flexible on dates, that way you should be able to pick something up for around $100 USD per person per night. The weakness of the USD does not help. The fiesta is the best party in the world, so everybody want to go, hence the prices go up as you have 20 people chasing every bed. But hell, running with the bulls is a once in a lifetime thing so it is worth spending some cash, to be able to get a few hours sleep before you risk life and limb. Best of luck